It is conventional to add a UV absorber or a UV scattering agent (e.g., zinc oxide, titanium dioxide) to a sunscreening cosmetic material in order to intercept the irradiation of the skin with ultraviolet light to thereby attain a high sun protection factor (SPF) value. For instance, Patent Literature 1 discloses a sunscreening cosmetic material containing octyl triazone and phenylbenzimidazole sulfonic acid in combination. In Patent Literature 2, the present applicant has proposed a water-in-oil emulsion type sunscreening cosmetic material composed of octocrylene, hydrophobicized titanium dioxide and/or zinc oxide, phenylbenzimidazole sulfonic acid and a salt formed by neutralizing phenylbenzimidazole sulfonic acid, as a material effective at shielding against ultraviolet light and highly stable in prevention/suppression of deterioration in smell with time.
The above mentioned phenylbenzimidazole sulfonic acid is one of few effective UV absorbers becoming water-soluble if neutralized with sodium hydroxide or triethanolamine into a salt, and is suitably blended in a water-containing cosmetic material. Phenylbenzimidazole sulfonic acid as a general-purpose UV absorber nevertheless has a problem in that discoloration occurs with time if the acid is used for a water-in-oil type sunscreening cosmetic material in combination with a hydrophobicized UV scattering agent (e.g., zinc oxide, titanium dioxide). In order to solve the problem, the present applicant has proposed in Patent Literature 3 a water-in-oil emulsion type sunscreening cosmetic material containing hydrophobicized titanium oxide and/or zinc oxide, phenylbenzimidazole sulfonic acid, triethanolamine, and trimethyl siloxysilicate in adjusted amounts.
Ultraviolet light is classified by wavelength into three types, ultraviolet A (UVA) at 320 to 400 nm, ultraviolet B (UVB) at 290 to 320 nm, and ultraviolet C (UVC) at 290 nm or below. Since most of UV absorbers are effective in a specified wavelength range, attempts are made to add two or more UV absorbers to a water-in-oil emulsion type sunscreening cosmetic material aiming at absorption of ultraviolet light in a wide range from UVA to UVB, and expecting the improvement in UV protection effect owing to the combined use of different UV absorbers.
In recent years, diethylamino hydroxybenzoyl hexyl benzoate and bis-ethylhexyloxyphenol methoxyphenyl triazone have been developed as a UVA absorber, and octyl triazone as a UVB absorber. Such substances are solid at room temperature, and need to be dissolved in an ester oil or a cinnamate UV absorber (such as octyl methoxycinnamate) in liquid form upon blending.